1. Field of the Invention
This invention relates to a method for manufacturing a friction material, and more particularly to a method for manufacturing a friction material of the resin mold type for use as brake pads for disc brakes and the like.
2. Description of the Prior Art
Friction materials containing a resin binder (hereinafter referred to as a resin mold lining) are usually made of an inorganic fiber-based friction material including a binder such as a phenolic resin. The conventional method for manufacturing the resin mold linings known as the dry method incorporates a powdered resin as binder.
Resin mold linings produced by the conventional dry method, are obtained in a process which comprises: mixing an asbestos fiber with a binder, graphite powder, a filler, and other ingredients in a mixing device such as a Nanta Mixer, thereby obtaining an asbestos mixture; preforming the asbestos mixture into a required shape under pressure at constant temperature; pressing and heating the preformed product in a metal mold while initially removing gas under reduced pressure; and completely curing the product in an oven, thereby obtaining the complete resin mold lining.
Typical components of conventional resin mold linings include: metallic powders such as ferric powder and copper powder; organic fiber such as rubber dust or cashew dust; and inorganic fillers such as barium sulfate. Preforming or forming machines such as a compression molding machine are commonly used in the process.
The conventional method described above has difficulties in that it is necessary to increase the amount of graphite powder so as to improve wear resistance, but the added graphite causes the friction coefficient, and thus the resistance against fading, to decrease. A need therefore continues to exist for a method that will permit adequate graphite loading while at the same time preserving desirable fade resistance characteristics.